Breaking A Barrier

Computer-on-a-chip may become even faster

Miraculous as it may be, the tiny silicon "chip" that is at the heart of today's electronics revolution has certain drawbacks. Crammed with thousands of individual circuits and components, this computer-on-a-chip is only about a quarter the size of a thumbnail. Yet despite the minuscule dimensions of these circuits, the time required for electric current to traverse them places a limit on how speedily the little computer can make its calculations.

Now the wizards at Bell Telephone Laboratories in Murray Hill, N.J., who launched the microelectronic age with the invention of the transistor 31 years ago, have broken that speed limit. Bell scientists have developed a way of at least doubling the velocity at which electrons race through tiny chips. Their feat could point the way to a whole new generation jf "smart," computer-run devices in the lome as well as in industry.

To make transistors and chips, scientists "dope" a semiconducting material ike silicon with impurities, creating regions that have either an excess or a deiciency of electrons--and thus are negatively (n zones) or positively (p zones) charged. If two n zones, say, are separated by a p zone, they act like an electronic switch, or transistor; a small voltage in the p zone controls fluctuations in the current flowing between the n zones. But every time an excess electron is released in the n zone to join the current flow, it leaves behind a positively charged spot. Because opposite charges attract, these spots act as obstacles, pulling at or even trapping the negatively charged electrons in the current, thus slowing its flow.

To create what in effect is an electron freeway without these obstructing potholes, Bell Physicist Raymond Dingle and his colleagues built a semiconductor made of extremely thin, alternate layers of aluminum gallium arsenide (which they doped) and gallium arsenide (which they left pure). They reasoned that any electrons donated by the impurity would tend to migrate to the adjoining undoped gallium arsenide layer because of their tendency to seek what physicists call a lower energy state. Explains the Australian-born Dingle: "It's rather like the inclination of water to flow downhill." The new design worked. Isolated from the obstructing impurities in the alternate layers, electrons flowed at unprecedented velocities through the gallium arsenide layers: nearly twice as fast at room temperatures, and as much as 20 times as fast at lower temperatures.

For the moment, the work remain at the experimental level. But Dingle see many practical future applications, ranging from stereo set components that require less energy to a new generation o high-speed computers and telephone transmission systems. Even more dazzling devices may be in the offing. At present semiconductors are flat; their electrons, for all practical purposes, flow in a single plane. But with the new layering technique, Dingle foresees three-dimensional devices in which electrons flow in all directions. That could make possible even tinier circuitry that would make today's minuscule computers look like veritable dinosaurs. -

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